Most of the cellphones and smartphones and alike mobile devices worldwide (hereinafter ‘mobile phones’ or simply ‘phones’), as well as other wireless devices feature a customized antenna, i.e., an antenna that is designed and manufactured ad-hoc for each device model. This is because each wireless device or, more specifically, each phone features a different form factor, a different radioelectric specification (e.g., the number and designation of mobile bands ranging for instance from GSM/CDMA 900/1800 to UMTS 2100 and the multiple LTE bands), and a different internal architecture. It is known that the relationship between antenna size and its operating wavelength is critical, and since many of the typical mobile operating wavelengths are quite large (e.g., on the order of 300 mm and longer for GSM 900 and other lower bands), fitting a small antenna inside the reduced space of mobile platform such as a smart phone is cumbersome. A typical available space inside a smart phone for an antenna is about 55×15×4 mm, which is much smaller than some of the longest operating wavelengths (e.g., below ⅙ of such a wavelength), and it is known that when antenna is made smaller than a quarter of a wavelength, both its impedance bandwidth and radiation efficiency rapidly diminish with size.
Owing to such constrains, antenna technology has evolved to provide complex antenna architectures that efficiently occupy and makes use of the maximum space available inside the mobile phone. This is enabled for instance by Multilevel (WO 0122528 A1) and Space-Filling (WO 0154225 A1) antenna technologies, which seek to optimize the antenna shape that, on a case-by-case basis, extracts the maximum radiation efficiency for each phone model.
While those technologies are flexible enough to provide an antenna solution for nearly every phone model and, therefore, have become mainstream technologies since about the beginning of the century, they still require the use of as much available space as possible inside the phone. More recently, some phones such as for instance the iPhone 4 and iPhone 5 series have introduced an antenna element that reuses an external metal frame mounted on the edge of the phone for radiation purposes. Those related solutions (hereinafter “metal frame antenna” or “MFA”) potentially benefit from minimizing the use of the internal space inside the phone as the metal frame is casted on the phone perimeter. Also, the available length on the perimeter can be used to embed a metal frame antenna sufficiently large to match about a quarter of the longest wavelength of the phone. Despite these advantages, such MFA solutions still present some drawbacks. Firstly, they are usually a single length element which matches eventually well one single wavelength but not the diversity of wavelengths that are available and needed in modern cellphones. Secondly, being an external element, its functioning is susceptible of being altered by the touch of a human user, causing a sever antenna impedance detuning or bandwidth reduction (see for instance the reported ‘antennagate’ problem with the iPhone 4 by N. Bilton, “The Check is in the Mail, From Apple,” the New York Times, Apr. 23, 2013).
FIG. 1 illustrates a typical problem related to the use of a metal frame as an antenna for a multiband system such as a cellphone. Curve (1) displays the achievable bandwidth for an example that includes a single strip frame 1 which is about the length of the upper edge of a phone (i.e., about 50 mm), as shown in FIG. 2. This curve is obtained by using a typical Electromagnetic CAD tool (e.g., IE3D) to compute the input impedance of an MFA mounted on a typical ground plane of a smartphone, and applying the achievable bandwidth equation as described in the following reference: Arthur D. Yaghjian, and Steven R. Best, “Impedance, Bandwidth, and Q of Antennas,” IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 4, April 2005, pp. 1298-1324.
While the achievable bandwidth function in FIG. 1 has a maximum in the lower frequency region of operation of a phone, e.g., 824-960 MHz, it severely decays in the upper 1710-2690 MHz range. Such a range, where for instance multiple 3G and 4G services are located, features a relative bandwidth of about 45%, while curve (1) shows that the maximum achievable bandwidth there is about a 10%. This is because being a single strip element, the frame enters into a second resonance mode at the upper range and this mode inherently features a small bandwidth.
This means that, while the usual criteria “make the antenna as large as possible to improve bandwidth” might be valid from a single-band system perspective, is no longer true for a multiband system: while the bandwidth looks optimum for the lower frequency region, it is far from optimum in the upper frequency region.
The problem is that this curve features a single peak, and changing the length of the strip apparently should not solve the problem. Owing to the scaling principle of Maxwell Equations, one can think that scaling down the strip would result in shifting the maximum of curve (1) to higher frequencies. And indeed such a maximum can become located at the upper region, but then one should expect a sever degradation in the lower frequency range as the peak moves away from such a region. A person skilled in the art may think that an MFA solution based on a single-strip is not appropriate for a multiband or multi-region system and therefore abandon this path of work or seek a multiband solution that combines a plurality of strips to accommodate the plurality of frequency regions and frequency bands of a phone.
It is the purpose of the present invention to provide an MFA antenna solution that fulfills the electromagnetic, radioelectric, mechanical and aesthetics requirements of a phone, particularly a smartphone or smartphone-like device. The present invention can also be extended to other wireless devices.